Scientists have made the extraordinary discovery in Chile of a hidden nursery where blue whales go in large numbers to rear their young and to feed.

The find, in the south of the country, will help researchers understand the behaviour and migration of blue whales, aiding conservation measures.

The iconic blue whale is the largest mammal on Earth and was driven to near-extinction by commercial whaling.

Details of the find are published in the scientific journal Biology Letters.

The researchers claim the area, located in a sheltered network of fjords surrounded by long-dormant volcanoes, is one of the most important blue whale (Balaenoptera musculus) feeding and nursing grounds yet discovered in the southern hemisphere.

Amazing discovery

They occur principally around the Gulf of Corcovado and the western coast of Chiloe island.

"It was amazing to discover something like this in 2003. One thinks this is not an era of discovery, but we still have much to learn," marine mammal ecologist Dr Rodrigo Hucke-Gaete, of the Universidad Austral de Chile, told BBC News Online.

The Chilean population could be pygmy blue whales

Dr Hucke-Gaete said it was possible that the whales were also giving birth in the area, but there was no evidence currently either way for this.

Scientists have little information on where blue whales breed, although it is thought to take place in tropical waters.

The nursing and feeding area was discovered following a blue whale study cruise along the Chilean coast in early 1997 organised by the International Whaling Commission (IWC).

Observers on the boats noticed that many whale sightings were occurring closer to shore than expected.

A follow-up boat trip into the fjords by whale expert Don Ljungblad and others found the site which was later identified as a whale nursing and feeding ground.

Mother's pride

Aerial and marine surveys in 2003 recorded at least 11 mother and calf pairings, suggesting the area was indeed being used to rear whale calves.

Dr Ken Findlay, of the University of Cape Town, South Africa, a co-author of the research, said the discovery of the site would give scientists easy access to the whales, helping conservation efforts.

It was amazing to discover something like this in 2003, one thinks this is not an era of discovery

Rodrigo Hucke-Gaete, Universidad Austral de Chile

But the findings appear to contradict traditional theories about the seasonal migration of blue whales.

They arrive in the tropics at the onset of winter, in order to give birth and breed. When spring arrives, they begin to migrate back to the Antarctic.

However, the blue whales at Chiloe-Corcovado were clearly spending summer near the tropics, in contrast to conventional wisdom.

Pygmy whales

One possibility is that the population found in the fjords of southern Chile are in fact pygmy blue whales (Balaenoptera musculus brevicauda), thought by some marine biologists to constitute a separate sub-species.

But the designation is highly controversial and not accepted by all whale experts. Pygmy blue whales are not thought to follow the same migration patterns as "true" blue whales.

The authors of the latest paper say it is too early to determine whether the Chilean whales belong to this sub-species and that genetic testing, satellite tracking and photogrammetry - the technique of measuring objects from images - is needed to say for certain.

Whaling reduced blue whale stocks to 3% of their original numbers

Andrew Read, assistant professor of marine conservation biology at Duke University, North Carolina, US, said he would not be surprised if migration patterns of blue whales were more complex than previously believed, because information on them is partly based on very old research.

"If we find large aggregations of blue whales outside their known habitats then that's good news because there are more than we thought," Dr Read added.

Another possibility - that the animals are capable of changing their habits - is not currently accepted by many marine biologists. But some scientists do consider this a plausible alternative.

The IWC estimates that there could be up to 1,400 individual Southern-Hemisphere blue whales left. This has risen from 500 around a quarter of a century ago, but the recovery has been proceeding slowly.

The Chilean National Environmental Agency has endorsed a proposal towards declaring the area a protected marine park.